The present invention relates generally to microwave substrates, and more particularly, to a method for bonding microwave substrates and silicon substrates that leverages silicon integrated circuit fabrication and silicon micromachining technologies.
Microwave communication devices such as satellites employ hybrid microwave circuits such as switching matrices and phased array antennas.
The microwave circuits can include miniature electromechanical (MEM) switches. Commonly, silicon-on-insulator techniques are used in integrated circuits. One method for forming silicon-on-insulator substrates is by the implantation of oxygen ions into a silicon wafer. In this process, oxygen ions are injected deep into the silicon wafer and the wafer is annealed under high temperature to form the buried SiO2 layer. Another method for forming silicon-on-insulator devices uses a first silicon wafer onto which the desired structures are etched. Then, in a post processing step, a second silicon wafer is bonded to the first wafer to enclose the structures. This post processing technique adds to the cost of the devices.
Another method for silicon-on-insulator bonding for two silicon wafers is to use a silicon dioxide bond between two silicon layers. Silicon dioxide is used to bond the two silicon wafers together.
One drawback to the above mentioned silicon-on-insulator methods is that they are intended for the high volume integrated circuit industry. The silicon-on-insulator wafers are extremely pure and thus very expensive. However, because MEMS technology has been highly developed, the cost of processing circuits using this technology is reduced.
It is commonly thought, however, that silicon-based MEMS fabrication processes are not amenable to the fabrication of microwave devices. Silicon has poor microwave properties and thus it was previously thought that the silicon substrate must be removed. Such devices are flip chip bonded to an insulating device where the silicon substrate can then be etched away. This adds to processing steps and increases the cost of the device.
The present invention therefore provides a methods for bonding a silicon substrate to a microwave substrate that will enable the use of mature silicon MEMS fabrication technology.
The present invention provides two alternative techniques to bonding silicon with a microwave substrate. The first method is a direct fusion bonding method wherein silicon is directly bonded with a microwave substrate. In the second method, an SiO2 layer is deposited on the silicon and microwave substrate and the bond is formed between the two SiO2 layers.
In one aspect of the invention, the direct fusion technique includes the steps of:
cleaning the microwave substrate;
cleaning the silicon substrate;
stacking the microwave substrate and the silicon substrate together to form a stack;
placing the stack in a furnace;
increasing the temperature of the furnace to a predetermined temperature at a predetermined rate; and
decreasing the temperature of the furnace to at a second predetermined rate.
In the indirect method, prior to the steps of stacking the microwave substrate and the silicon substrate, an SiO2 layer is sputtered onto the silicon layer and the microwave substrate.
One advantage of the invention is that the present invention enables the fabrication of low insertion loss microwave circuits and MEMS devices on silicon-based technology. The present invention also alleviates the need for post processing of the silicon wafers after MEMS processing.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.